Low frequency omnidirectional projectors are an integral part of current sonar systems, including buoys, tethered projectors and mounted systems in submarines. Small diameter systems are used to project a very broadband, very high source level acoustic signal into the water to be as effective as possible. One type of projector utilizes an omnidirectional signal, of which ring shaped transducers are used in a radial or “hoop” mode by expanding and contracting the ring inner diameter and outer diameter uniformly and in phase. This resonance is excited by either poling through the ring wall thickness or circumferentially around the ring tangential to the ring's outer surface.
With conventional ceramic ring transducers, such as lead zirconate titanate (PZT-4, Navy Type 1), the piezoelectric projector is relatively large in volume and requires a high power input, due to the relatively low electromechanical coupling, high stiffness and low piezoelectric coefficients of the ceramic material. This strains the design requirements for packaging length, weight, and battery power, leaving insufficient reserve for the insertion of mobility and full duplex capability for improved countermeasure effectiveness in new torpedo applications. The large volume of these ceramic transducers is due in part to the need of multiple ceramic component rings, each of which is a different size in the transducer, which are necessary to provide the bandwidth required of the transducer. Each of these rings requires a separate drive component, which further increases the footprint of the projector system.
In an attempt to solve the problems associated with ceramic ring transducers, single crystal ring transducers were formed using wedge shaped bodies of single crystal material, for example Pb(Mg1/3Nb2/3)1-xTixO3 (PMN-PT), in a ring-approximation oriented along the <001> crystallographic orientation, glued together with electrode shims in between. The angled sides of the wedge shaped bodies are electroded with a conductive layer such as gold, and the bodies are glued together with conductive shims between each wedge shaped body.
The <001> orientation is traditionally considered the highest piezoelectric mode, providing the highest electromechanical coupling, and therefore largest bandwidth, for the single crystal material. This orientation eliminated the assumed non-uniform angular pressure field pattern from a circular single crystal ring, due to the anisotropy of the crystal around the circumference. In this wedge approach, the crystal wedges were poled such that the applied field was tangential to the ring, as with the circular ceramic rings.
Single crystal wedges are used because the anisotropy of PMN-PT could potentially reduce the omnidirectional behavior of the overall ring, and the wedges operate close to the 33-mode which provides an electromechanical coupling near 90%. Based on the height of the rings, the pure 33-mode is not generally achieved because the aspect ratio (width to thickness) is not above 3:1, and the width to wedge height is not above 3:1. Also, the overall diameter of the ring must be increased to achieve lower frequencies, and currently there is a practical limit as to how large a single ring of the single crystal can be formed.
Therefore, what is needed is a single crystal ring resonator having increased bandwidth allowing for more flexibility in duplexing, source level and/or better resolution, which does not suffer from the drawbacks of the prior art.